With the advent of optical fiber telecommunications technology, the delivery of broadband services such as high definition video to subscriber premises is now possible. Accordingly, there has been much consideration of fiber optic subscriber loop architectures. The subscriber loop refers to the portion of a telecommunications network which goes directly into the subscriber premises. (See, e.g., P. Kaiser, "Single mode fiber technology in the subscriber loop environment," in Proc. OFC 1987, Reno, January 1987, paper MD1; Linnell, L. R., "A wide-band local access system using emerging technology components," IEEE J. Selected Areas Comm., SAC-4, July 1986, pp. 612-618; P. Cochrane et al., "Local line single mode optics--viable options for today and tomorrow," IEEE J. Selected Areas Comm., SAC-4, December, 1986, pp. 1438-1445; B. S. Mullinix, "Residential fiber optic subscriber loops--information pipeline or technology pipedream" ibid., pp. 1446-1450).
Virtually all fiber optic subscriber loop designs utilize some form of multiplexing over at least a portion of the signal path, with information to and from different subscribers being multiplexed together on the same fiber. Thus, the choice of multiplexing technique is a fundamental part of the subscriber loop architecture, and it will have an important impact on cost, complexity, power requirements and flexibility.
A wide variety of multi/demultiplexing techniques are available for combining and separating different subscriber channels on an optical fiber. Among the possibilities are high speed time division multiplexing (TDM), various forms of wavelength division multiplexing (WDM), code division multiplexing, polarization multiplexing, and subcarrier frequency multiplexing. At present, the most common multiplexing technique is TDM. TDM techniques have been used extensively in wideband subscriber loop prototypes (see, e.g., Linnell, L. R., "A wide-band local access system using emerging technology components," IEEE J. Selected Areas Comm., SAC-4, July 1986, pp. 612-618). Although transmitted using optical fibers, the processing and routing of such TDM signals is generally accomplished utilizing electronics. The TDM approach benefits from the relative maturity of electronics technology, though a fundamental drawback is the need for controlled environmental vaults, power back-up and maintenance at the processing site.
Consider, for example, a double-star subscriber loop architecture wherein there is a single Central Office, a plurality of Remote Nodes connected via optical fibers to the Central Office, and a plurality of subscribers connected via optical fibers to each Remote Node. The use of electronic TDM at the Remote Nodes involves optical-to-electrical and electrical-to-optical conversion of all signals coming into or out of the Remote Nodes, as well as the need for controlled environmental vaults, power back-up and maintenance at the Remote Nodes.
Of the many alternatives to TDM that use optical rather than electronic multiplexing, one of the most viable options is WDM (See, e.g., H. Toba et al., "A conceptual design on frequency-division-multiplexing distribution systems with optical tunable filters," IEEE J. Selected Areas Comm. SAC-4 December 1986, pp. 1458-1467; D. B. Payne and J. R. Stern, "Transparent single-mode fiber optical networks," J. Lightwave Tech., LT-4, July, 1986, pp. 864-869). Compared to TDM, WDM can have significant advantages. In particular, wavelength multiplexed channels can be separated and combined passively, independently of the format and bit rate of the data being transmitted.
However, prior art subscriber loop architectures based on WDM have suffered from a number of drawbacks. The most basic technological requirement of any multi-channel WDM system is that the emission wavelengths of the light sources be coordinated and stabilized to maintain a sufficient wavelength separation between the channels. Unfortunately, the emission wavelengths of conventional laser diodes are somewhat unpredictable, owing to variations in operating temperatures, manufacturing procedures and other factors. This uncertainty complicates efforts to perform WDM with a large number of channels as is required in a subscriber loop architecture, particularly if the conventional laser diodes are in many disjoint locations such as at different subscriber premises.
Another problem with prior art subscriber loop architectures based on WDM is the inefficient manner in which channels are distributed to the individual subscribers. Such systems (see, e.g., D. B. Payne and J. R. Stern, "Transparent single-mode fiber optical networks," supra) transmit a fraction of the power from many wavelength channels to each of a plurality of subscriber stations. A filtering operation is then performed at each subscriber station in order to select the proper channel from all the channels received. This arrangement has drawbacks from the point of view of privacy, from the point of view of power budget (i.e. only a limited number of subscriber stations can be serviced when power is divided in this manner), and from the point of view of modularity of design (i.e. different subscriber stations require differently tuned filters, rather than all subscriber stations utilizing the same equipment).
Accordingly, it is an object of the present invention to provide a fiber optic subscriber loop architecture based on WDM which overcomes the shortcomings of prior art fiber optic subscriber loop architectures.
It is a further object of the present invention to develop a fiber optic subscriber loop architecture based on a laser sharing scheme (see, e.g., Cheng, U.S. Pat. No. 4,705,350, Cheng et al., U.S. Pat. No. 4,658,394, Albanese et al., U.S. Pat. No. 4,712,859 and Personick, U.S. Pat. No. 4,642,804) so that a large number of wavelength channels, and thus a large number of subscribers, can be accommodated without problems resulting from the instability of and lack of coordination between the wavelengths emitted by conventional laser diodes.
It is yet a further object of the invention to provide a fiber optic subscriber loop architecture which utilizes no active components outside the central office, which is totally transparent to the bit rates and formats of the data being transmitted, and which is modular in design.